Ventilatory stabilization technology

ABSTRACT

A system for reducing central sleep apnea (CSA) is described in which certain methods of increasing a patient&#39;s rebreathing during periods of the sleep cycle are used. By increasing rebreathing during periods of overbreathing, the over-oxygenation which typically results from the overbreathing period can be reduced, thus reducing the compensating underbreathing period and effectively reducing the loop gain associated with the central sleep apnea. Nasal occlusion and a leak resistant oral interface provide control for gas leaks from a patent interface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.09/498,504 filed Feb. 3, 2000, now U.S. Pat. No. 6,752,150.

BACKGROUND OF THE INVENTION

Central sleep apnea is a type of sleep-disordered breathing that ischaracterized by a failure of the sleeping brain to generate regular,rhythmic bursts of neural activity. The resulting cessation of rhythmicbreathing, referred to as apnea, represents a disorder of therespiratory control system responsible for regulating the rate and depthof breathing, i.e. overall pulmonary ventilation. Central sleep apneashould be contrasted with obstructive sleep apnea, where the proximatecause of apnea is obstruction of the pharyngeal airway despite ongoingrhythmic neural outflow to the respiratory muscles. The differencebetween central sleep apnea and obstructive sleep apnea is clearlyestablished, and the two can co-exist. While central sleep apnea canoccur in a number of clinical settings, it is most commonly observed inassociation with heart failure or cerebral vascular insufficiency. Anexample of central sleep apnea is Cheyne-Stokes respiration.

The respiratory control system comprises a negative feedback systemwherein a central pattern generator creates rhythmic bursts of activitywhen respiratory chemo-receptors sensing carbon dioxide, oxygen and pHare adequately stimulated (FIG. 1). While this neural output of thebrainstem central pattern generator to the respiratory muscles derivesfrom a neural rhythm generated intrinsically by the central patterngenerator, the generator becomes silent if the feedback signals, relatedto arterial P_(CO2) and P_(O2), are not sufficiently intense. In otherwords, the respiratory rhythm is generated by a conditional centralpattern generator which requires an adequate input stimulus derived fromperipheral chemoreceptors sensing arterial P_(CO2) and P_(O2) fromcentral chemoreceptors sensing brain P_(CO2)/pH. Furthermore, theintensity of neural activity generated by the respiratory centralpattern generator depends directly upon the arterial P_(CO2) inverselyon the arterial P_(O2). Thus, the central and peripheral chemoreflexloops constitute a negative feedback system regulating the arterialP_(O2) and P_(CO2), holding them constant within narrow limits (FIG. 1).

This normal regulation of arterial blood gases is accomplished by astable ventilatory output of the respiratory central pattern generator.By contrast, central sleep apnea represents an instability of therespiratory control system. The instability can arise from one of twomechanisms, namely: (1) intrinsic failure of the respiratory centralpattern generator in the face of adequate stimulation by respiratorychemoreceptors; or (2) lack of adequate stimulation of the centralpattern generator by respiratory chemoreceptors. The former is referredto as the “intrinsic instability” and the latter is referred to as the“chemoreflex instability.” Theoretically, both mechanisms can co-exist.The common form of central sleep apnea is thought to be caused by thechemoreflex instability mechanism.

The chemoreflex control of breathing might exhibit instability eitherbecause the delay of the negative feedback signal is excessively long orbecause the gain of the system is excessively high. Current evidenceindicates that the latter constitutes the principal derangement incentral sleep apnea caused by heart failure. Specifically, the overallresponse of the control system to a change in arterial P_(CO2) isthree-fold higher in heart-failure patients with central sleep apneathan in those having no sleep-disordered breathing. This increased gainprobably resides within the central chemoreflex loop; however, high gainof the peripheral chemoreflex loop cannot be excluded. Accordingly, thefundamental mechanism of central sleep apnea is taken to be high loopgain of the control system, which results in feedback instability duringsleep.

Central sleep apnea causes repeated arousals and oxyhemoglobindesaturations. Although firm evidence linking central sleep apnea tomorbidity and mortality is lacking, a variety of evidence leads to theinference that central sleep apnea may promote cardiac arhythmias,strokes, or myocardial infarctions. The repeated nocturnal arousals arelikely to impair daytime cognitive function and quality of life. Notreatment has become established as being effective for central sleepapnea. Stimulating drugs such as theophyline may be helpful, andcarbonic anhydrase inhibitors may relieve central sleep apnea in normalssleeping at high altitude. Nasal continuous positive airway pressure maydirectly or indirectly improve ventilatory stability. Increasinginspired fractional concentration (F) of oxygen in the inspired gasgenerally does not eliminate central sleep apnea, whereas increasinginspired F_(CO2) (F_(|CO2)=0.01–0.03) promptly eliminates central sleepapnea. However, long-term exposure to high F_(|CO2) would appear to bean undesirable long-term therapy.

SUMMARY OF THE PRESENT INVENTION

The present invention is a method for varying the efficiency ofpulmonary gas exchange by using a controlled amount of rebreathingduring certain periods of the central sleep apnea respiration cycle soas to counteract the effects of the transient excessive ventilation onthe level of carbon dioxide and oxygen in the lungs and in the arterialblood. In effect, this strategy is an attempt to stabilize breathing byminimizing oscillations in the feedback variables.

The invention counteracts periodic breathing due to central sleep apneaby decreasing loop gain of the respiratory control system. In oneembodiment, the invention dynamically modulates efficiency of pulmonarygas exchange in relation to pulmonary ventilation. When pulmonaryventilation is stable at resting values, the performance of the systemis unchanged. However, during a period of hyperpnea, i.e. whenventilation increases transiently to supra-normal levels, the system ismade more inefficient, thus decreasing loop gain and stabilizing thesystem.

Rebreathing can be used to increase the inspired percentage carbondioxide and reduce the inspired percentage oxygen just before or duringthe period of overbreathing. In one embodiment, the patient'sventilation is continuously monitored and analyzed in real time so thatthe ventilation periodicities of the central sleep apnea breathing canbe detected and the inspired carbon dioxide and oxygen concentrationsadjusted appropriately by varying the amount of exhaled gas that isreinspired.

In another embodiment of the present invention, a rebreathing apparatusis a part of a nasal continuous positive airway pressure (CPAP) system.The use of continuous positive airway pressure may have a beneficialeffect on cardiac function in patients with congestive heart failure. Inthe future it is likely that patients with congestive heart failure willreceive nasal CPAP for treatment of the heart failure. Central sleepapnea may not immediately disappear upon administration of conventionalnasal CPAP therapy as central sleep apnea respiration is basically of anon-obstructive origin. However, over a period of about four weeks thedegree of heart failure improves; thus, the resulting central sleepapnea respiration may be relieved by the continuous positive airwaypressure. This is described in the papers, Naughton, et al., “EffectiveContinuous Positive Airway Pressure on Central Sleep Apnea and NocturnalPercentage Carbon Dioxide in Heart Failure,” American JournalRespiratory Critical Care Medicine, Vol. 1509, pp 1598–1604, 1994;Naughton, et al., “Treatment of Congestive Heart Failure and CentralSleep Apnea Respiration during Sleep by Continuous Positive AirwayPressure,” American Journal of Critical Care Medicine, Vol. 151, pp92–97, 1995; and, Naughton, et al., “The Role of Hyperventilation in thePathogenesis of Central Sleep Apneas in Patients with Congestive HeartFailure,” American Review of Respiratory Diseases, Vol. 148, pp 330–338,1993.

It is desirable to have a prompt elimination of the central sleep apnearespiration because the resulting daytime sleepiness and impairedcognition resulting from repeated arousals impair the patient's qualityof life. Immediately relieving central sleep apnea breathing during theCPAP treatment would have the advantage that the patient wouldexperience a better sleep and would be more rested. This in turn wouldenhance compliance with the CPAP treatment program. Conventional nasalCPAP provides no immediate relief of central sleep apnea respiration andresulting arousals.

A conventional CPAP system is modified in one embodiment of the presentinvention to allow a controlled amount of rebreathing during a portionof the central sleep apnea respiration cycle. In this embodiment, avalve is used to control the amount of rebreathing. When the valve isclosed, rebreathing occurs and when the valve is open no rebreathingoccurs. A computer connected to a flow meter can be used to detectperiodicities in the central sleep apnea respiration cycle. The computercan then control the valve to open and close. Nasal occlusion incombination with an oral appliance may be used to guarantee controlledre-breathing.

Another embodiment of the present invention concerns a passivelow-bias-flow device for treating central sleep apnea. This apparatusincludes a gas-supply means, such as a blower, and a patient interfacethat is fitted to a patient's airway. The gas-supply means is adjustedso that air flow from the gas-supply means is such that for thepatient's normal breathing, the gas flow supplied by the gas-supplymeans is sufficient to prevent a significant amount of the patient'sexhaled gases from flowing retrograde into a tube between the gas-supplymeans and the patient interface. During periods of increased breathingpreceding or following central sleep apnea, the preset air flow is suchthat some of the patient's exhaled gases flow retrograde into the tube.Some of the exhaled gases flowing retrograde into the tube will berebreathed by the patient. Thus, during periods of overbreathingassociated with central sleep apnea, there will be some rebreathing ofgases containing a higher F_(CO2) and a lower F_(O2) than room air. Notethat conventional CPAP systems are set such that there is no retrogradeair flow any time in the sleep cycle.

Yet another embodiment of the present invention is a method foradjusting an apparatus comprising a gas-supply means, a patientinterface and a tube between the patient interface and the gas-supplymeans. In this method, the patient interface is fitted to the patient'sairway. The supply of gas from the gas-supply means is set high enoughthat during the patient's normal breathing, the gas flow supplied by thegas-supply means is sufficient to prevent a significant amount of thepatient's exhaled gases from flowing retrograde into the tube, but setlow enough that during periods of increased breathing increased withcentral sleep apnea, some of the patient's exhaled gases flow retrogradeinto the tube.

Still another embodiment of the present invention concerns an apparatusfor treating central sleep apnea wherein the supply of gas from agas-supply means has a varying gas pressure that changes at differenttimes during the patient's sleep cycle. In this way, rebreathing can beincreased. For example, in one embodiment, the gas pressure from theblower is decreased during periods of increased breathing associatedwith central sleep apnea so that some of the patient's exhaled gasesflow retrograde between the patient interface and the blower. Thisapproach is less advantageous because users often find the varyingpatient interface pressure to be annoying. Also, varying of the patientinterface pressure can affect the internal dead space in a mannercounter to the rebreathing effect.

The general approach is that the blower pressure is set at a minimumlevel that eliminates all evidence of upper airway obstruction, or at alevel deemed appropriate for treating heart failure. The bias flow isthen reduced to a level that eliminates central sleep apnea withoutincreasing the external dead space during unstimulated breathing. Thebias flow can then be fixed at this level or varied systematicallywithin or between cycles of periodic breathing.

BRIEF DESCRIPTION OF THE DRAWINGS

There will now be described preferred embodiments of the invention, withreference to the drawings, in which:

FIG. 1 is a diagram illustrating central sleep apnea;

FIG. 2 is a diagram illustrating one embodiment of the rebreathingapparatus of the present invention;

FIG. 2A is a diagram illustrating use of the embodiment of FIG. 2 with adental appliance;

FIGS. 2B and 2C and 2D illustrate two embodiments of an oral applianceof FIG. 2A;

FIG. 3 is a diagram illustrating central sleep apnea respiration;

FIG. 4A is a diagram of one embodiment of the present invention using apassive loop gain modulation for ventilization stabilization using asingle pre-set gas flow pressure from a blower;

FIG. 4B is a diagram of an alternate embodiment of the system of FIG. 4Ausing a flow meter and a computer;

FIG. 5 is a diagram of one embodiment of the present invention whichuses computer control of the blower pressure to modify the vent pressurefrom the blower during certain periods of a sleep cycle;

FIG. 6 is a diagram of an embodiment of the present invention which usescomputer control of a dead space attached to valves so as to causerebreathing during certain periods of a sleep cycle;

FIG. 7 is a diagram of one embodiment of the present invention using arecirculator to increase rebreathing during certain periods of a sleepcycle;

FIGS. 8A–8F are diagrams depicting air flow accorded in tubingconnecting between the blower and the mask;

FIG. 9 depicts the changes in V_(ret) and V_(wash) that occur whenpulmonary ventilation is stimulated by increasing arterial P_(CO2);

FIGS. 10, 11 and 12 are diagrams that illustrate the dependence ofV_(ret), V_(ED) and T_(FRAC) on V _(E);

FIG. 13 is a diagram that illustrates the relationship of V _(A) and V_(E) at the four levels of V _(B);

FIG. 14 is a diagram illustrating the general dependence of the loopgain on the ratio log V _(E)/V _(A);

FIG. 15 is a diagram that illustrates the breathing air flow in the tubeof a conventional CPAP system;

FIG. 16 is a diagram that illustrates the normal breathing flow in thetube of the

embodiment of FIG. 4A;

FIG. 17 is a diagram that illustrates overbreathing flow in the tube inthe embodiment of FIG. 4A;

FIG. 18A is a diagram of an embodiment of the present invention in whichthe size of the exit tube of the mask is varied slowly over thepatient's sleep cycle;

FIG. 18B is a graph illustrating one example of changing of the exithole size during the night, for the apparatus of FIG. 18A; and

FIG. 19 is a diagram of an alternate embodiment using the blower outputas an active control device to adjust the level of rebreathing by apatient.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 2 is a diagram illustrating the rebreathing apparatus of one activecontrol embodiment of the present invention. In this embodiment, acontinuous positive airway pressure apparatus including blower 20, tube22 and patient interface 24 is used. Patient interface 24, for example amask or oral interface, preferably produces an airtight tight seal tothe face for use in the continuous positive airway pressure treatment. Adiscussion of continuous positive airway pressure and a preferredcontinuous positive airway pressure apparatus is described in Remmers,et al. U.S. Pat. No. 5,645,053, “Auto-CPAP Systems and Method forPreventing Patient Disturbance Using Airflow Profile Information.” Inconventional CPAP, a blower is used to maintain a relatively highconstant pressure in a mask and to provide a bias flow of fresh air fromthe blower out the mask.

In one embodiment of the present invention, tube 26 is connected to theexhaust port 31 of the patient interface and conducts gas to thevariable resistor 28. Alternatively, the valve can be located on theexhaust port of the patient interface. Tube 22 is used as a dead spacefor rebreathing during some periods of the central sleep apnearespiration. When the valve 28 is open, no rebreathing occurs becauseall the exhaled gas is carried out tube 26 through valve 28 by the biasflow before inspiration occurs. When valve 28 is closed, the bias flowceases and no expired air is conducted through tube 26. In this case,some partial rebreathing occurs because the expired air is conductedretrograde up tube 22 to the blower. The gases in the tube 26 have ahigher concentration of carbon dioxide and a lower concentration ofoxygen than room air. When the patient inspires, gas is conducted fromthe blower to the patient and the previously expired gases are inhaledby the patient.

Normally, the bias flow of gas from the blower through the patientinterface and out port 30 would be adequate to completely purge thesystem during the expiratory phase of the respiratory cycle so that nogas expired by the patient remains in the system. Thus, the gas inspiredby the patient had a composition of room air (O₂ concentration 21%; CO₂concentration about 0%). Conversely, if the bias flow is reduced to zeroby completely occluding port 30 with valve 28, the gas exhaled by thepatient would fill the tube 22 connecting the patient interface to theblower. Such expired gas would typically have a carbon dioxideconcentration of 5% and an oxygen concentration of 16%. Upon inhalation,the patient would first inspire the high carbon dioxide, low oxygenmixture filling the tube, followed by inhalation of room air from theblower. Depending upon the length of the tubing this mixture couldamount to rebreathing of 20 to 60 percent of the tidal volume. Byvarying the exhaust port outflow resistance, the degree of rebreathingbetween these limits can be varied and the inspired concentration ofcarbon dioxide and oxygen can be manipulated. In one embodiment, flowmeter 32 connected to computer 34 is used to detect the flow of gases toand from the blower 20. The computer 34 is used to identify theperiodicities in pulmonary ventilation caused by the central sleep apnearespiration and to control the valve 28 to cause rebreathing duringcertain periods of the central sleep apnea cycle.

The gas flow from the blower comprises the bias flow (patient interfaceexit flow+leak flow) plus the respiratory airflow. The computer monitorsthis flow and calculates the bias flow, leak flow, retrograde flow,retrograde expired volume and wash volume.

A computer 34 can detect the amplitude of the central sleep apnea cycleand to adjust the resistance of the valve 28 according. For example, ifthere are large variations in pulmonary ventilation during the centralsleep apnea cycle, the valve 28 can be completely closed during theoverbreathing period. If there are small variations in pulmonaryventilation during the central sleep apnea cycle, the valve 28 can bepartially open during the overbreathing period. Thus, a higher level ofrebreathing will occur when the variation in pulmonary ventilationduring the central sleep apnea cycle is high than will occur when thevariation in pulmonary ventilation during the central sleep apnea cycleis low.

Because of the low impedance of the CPAP blower 20, variations of theresistance in the outflow line cause very little change in patientinterface pressure. Accordingly, the full range of variations in outflowresistance can be made without producing significant deviations in thedesired CPAP patient interface pressure.

The flow meter 32 and computer 34 can quantitate the level of pulmonaryventilation. For example, the ratio of breath volume to breath periodgives an indication of the level of the instantaneous pulmonaryventilation. Other indices such as mean or peak inspiratory flow ratecould also be used.

FIG. 3 shows an idealized diagram of the periodicities of theoverbreathing and underbreathing during central sleep apnea respiration.This diagram shows the regions of overbreathing 50 a and the regions ofunderbreathing 50 b compared to the moving time average of ventilation.The computer system will be able to determine the periodicities of thecentral sleep apnea breathing. Typically, there is about a 50–60 secondperiodicity to the overbreathing and underbreathing in the central sleepapnea breathing.

A number of techniques are used to control the degree and timing ofrebreathing with the valve 28 in order to eliminate central sleep apnea.One way of controlling rebreathing so as to reduce the central sleepapnea respiration is to anticipate the different cycles in the centralsleep apnea respiration. For example, looking at FIG. 3, at time A, thesystem will anticipate a period of overbreathing and thus beginrebreathing by closing valve 28 as shown in FIG. 2. By the timeoverbreathing portion 50 a occurs, there is some level of rebreathing.Because of this, pulmonary gas exchange becomes less efficient duringthe period of overbreathing and, thereby, the resulting rise in lungoxygen and fall in lung carbon dioxide will be less. As a result, thelevel of oxygen in the blood does not get too high and the level ofcarbon dioxide does not get too low. This stabilizes the oxygen andcarbon dioxide pressures in the arterial blood and thus will reduce theamplitude of subsequent underbreathing or the length of the apnea. Attime B, the system will anticipate an underbreathing cycle by openingthe valve 28 and rebreathing will no longer occur. The apparatus of thepresent invention can reduce central sleep apnea rebreathing (line 50)to a lower level as shown in dotted line 60 in FIG. 2. Time A and time Bfor the beginning and end of the rebreathing can be determined by thecomputer 34 shown in FIG. 2.

FIG. 4A is a diagram that illustrates a passive loop gain modulationsystem for use in the present invention. FIG. 4A depicts a system usinga gas-supply means such as the air blower 60 connected to a length ofinput tubing 62 and then to a patient interface 64. This system uses asimple fixed exit port for the patient interface. A tubing volumegreater than that normally used with obstructive sleep apnea can be usedwith the present invention. For example, a ten-foot rather than six-foottubing can be used. The blower 60 preferably has a very low impedance.That is, changes in the air flow do not significantly change the airpressure supplied by the blower. This can help maintain a relativelystable patient interface pressure even as the tube flow becomesretrograde.

Additionally, in one embodiment, the air blower is able to supply airpressure much lower than conventional CPAP blowers. In one embodiment,the air blower can be adjusted to supply pressures below 4 cm H₂O(preferably 2 cm H₂O or below). The ability to supply such smallpressures allows for the retrograde flow as discussed below. The patientinterface is fitted about the patient's airway. During normal breathing,the air supplied from the blower 60 and tube 62 to the patient interface64 does not cause any rebreathing because any exhaled air will beflushed before the next inhale period. During periods of heavybreathing, the preset gas flow pressure is set so that enough exhaledair flows retrograde into the tube such that during the next inhaleperiod some expired gas is rebreathed. In this embodiment, theoverbreathing occurs during certain periods of the sleep cycleassociated with central sleep apnea. Rebreathing during periods ofoverbreathing during central sleep apnea tends to reduce the resultingspike in the blood oxygen level. Thus, the period of underbreathingfollowing the overbreathing in the central sleep apnea sleep cycle willalso be reduced.

The alternating periods of under- and overbreathing are reduced by therebreathing which takes place during the periods of overbreathing. Therebreathing attenuates the arterial blood oxygen spike and the reductionin arterial P_(CO2) caused by the overbreathing. Thus, there is lessunderventilation when the blood reaches the chemoreceptors. Thus, theamplitude of the periodic breathing is reduced.

The embodiment of FIG. 4A is different than the conventional CPAP inthat the preset gas flow pressure is lower and/or the patient interfaceexit hole is smaller than that used with conventional CPAP systems. Byreducing the gas flow pressure from the typical CPAP gas flow pressures,and/or reducing the patient interface exit hole size, the retrogradeflow during the overbreathing periods is produced.

The system of FIG. 4A has the advantage that it does not require activecontrol of the blower pressure. The patient can be checked into a sleepcenter and the correct blower pressure and patient interface exit holesize set. Thereafter, the system can be placed on the patient's airwayevery night without requiring an expensive controller-based system. Thepreset blower gas pressure depends upon the air flow resistance causedby the patient interface 64, normal exhale pressure and theoverbreathing exhale pressure. If the gas-supply pressure system is anair blower 60, then by modifying the revolutions per minute of the airblower, the preset gas flow pressure can be set.

The air supply pressure for patients with central sleep apnea butwithout obstructive sleep apnea can be set at a relatively low levelsuch as below 4 cm H₂O. The normal patient interface exit holes producethe desired effect at these pressures. The end-tidal F_(CO2) andinspired F_(CO2) can be monitored by a CO₂ meter 65 with an aspirationline connected to the patient interface. Importantly, all mouth leaksshould be eliminated by using a leak resistant patient interface inorder to have expired gas move into the tubing 62. This can be achievedby applying a chin strap, or by using an oral appliance 25 (FIG. 2A)such as a full arch dental appliance applied to the upper and lowerteeth, or both. An alternative approach to difficult mouth leaks is touse a full face mask covering the mouth as well as the nose. This meansthat expired gas emanating from the nose or the mouth will travelretrogradely up the tubing 62 toward the blower. While it is importantthat leaks between the patient interface and the patient be minimized,it is also important that as much as possible of the exhaled air of thepatient be conserved and made available for re-breathing. Hence, if thepatient interface connects to the nose, then the mouth passageway shouldbe blocked, and if the patient interface connects to the mouth, then thenasal passageway should be blocked. In either case, leaks through theunused passageway should be minimized.

Examples of an oral appliance 25 are illustrated in more detail in FIG.2B and FIG. 2C and FIG. 2D. In FIG. 2B, the oral appliance 25 is adental appliance. In FIG. 2B, the oral appliance 25 is designed forfitting within the teeth and has an upper tray 25A that fits between thelips and teeth of a patient, and a lower tray 25B that provides asealing surface for the lips to rest on. An opening 25C in the center ofthe oral appliance 25 of FIG. 2B communicates with a CPAP hose connector25D to provide CPAP pressure delivery. The oral appliance 25 of FIG. 2Cis fitted to a patient's mouth directly onto the lips, without using theteeth. The oral appliance 25 of FIG. 2C and FIG. 2D is held on a patientwith a mask 27 that fits around a patient's airway and is secured withthe use of straps and a pad 29A at the back of the patient's head. Atube 29B with normal bias ports 29C blocked, and low-flow bias flow port29D, connects to the CPAP apparatus through CPAP connection 29E. Thelength of the tube 29B allows for a controlled amount of rebreathing.

A feature of the mode of action of the technology described in thispatent document relates to the behaviour of the system duringhyperventilatory periods. At these times, when such a hyperventilatoryphase occurs, the patient generates a large tidal volume and shortduration of expiration. Together, these induce rebreathing of expiredgas that has flowed retrogradely into the CPAP conduit 29B connectingthe CPAP blower to the patient interface such as oral appliance 25.Patients with central sleep apnea using Low Flow CPAP nightly in thehome may find that, during periods of hyperventilation, mouth leaks mayoccur of sufficient magnitude to vitiate the rebreathing of exhaledgasses. For such patients, it is preferable to use a dental appliance 25to apply CPAP pressure to the mouth together with nasal occlusion toeliminate leaks from the nose. Data from studies on patients using adental appliance and nasal occlusion revealed that the therapy waseffective in resolving the central sleep apnea and that during hyperpnicphases no leak of exhaled gasses occurred. For effective application ofLow Flow CPAP an oral interface, such as the oral appliance 25, shouldbe used in combination with nasal occlusion. Nasal occlusion may beobtained through plugs inserted in the nostrils or an external U-shapedclamp 29F (FIG. 2D) similar to what would be used by a swimmer.

If the patient has an element of obstructive sleep apnea, the patientinterface pressure is increased progressively until all evidence ofupper airway obstruction is eliminated. If the patient is receivingnasal CPAP as treatment for heart failure, patient interface pressure isset at the desired level (typically 8–10 cm H₂O). The bias flow (patientinterface hole size) can then be reduced until central sleep apnea iseliminating without adding dead space.

For patients with heart conditions, the patient interface pressure canbe set at the valve suggested by the literature (typically about 10 cmH₂O). Then the bias flow is adjusted.

The flow through tube 62 depends upon the difference in pressure betweenthe blower pressure (i.e., pressure at the outlet of the blower) andpatient interface pressure. Blower pressure is set by the revolutionsper minute (RPM) of the blower and will be virtually constant becausethe internal impedance of the blower is very low. When no respiratoryairflow is occurring (i.e., at the end of expiration), patient interfacepressure is less than blower pressure by an amount that is dictated bythe flow resistive properties of the connecting tube and the rate ofbias flow. This is typically 1–2 cm of water pressure difference whenbias flow is at 0.5–1.5 L/sec. When the patient interface is applied tothe patient and the patient is breathing, patient interface pressurevaries during the respiratory cycle depending upon the flow resistanceproperties of the connecting tube and the airflow generated by thepatient. During inspiration the patient interface pressure drops,typically 1–2 cm of water, an during expiration pressure may risetransiently a similar amount. During quiet breathing the peak-to-peakfluctuations in patient interface pressure are less than during heavybreathing or hyperpnea.

Thus, during quiet breathing the patient interface pressure rises duringexhalation and this reduces the driving pressure difference between theblower and the patient interface, thereby reducing flow in the tube. Ifthe expired tidal volume increases, however, peak expiratory flow willincrease and this will be associated with a further increase in patientinterface pressure. If patient interface pressure increases to equalblower pressure, flow in the tube will stop. When patient interfacepressure exceeds blower pressure, flow in the tube will be in aretrograde direction, i.e., from the patient interface to the blower.Such retrograde airflow will first occur early in expiration and thevolume of air which moves into the connecting tube will be washed outlater in expiration as patient interface pressure declines and flow fromthe blower to the patient interface increases. However, if bias flow islow and the tidal volume is large, a large amount of retrograde flowwill occur and a large volume of expired gas will move into the tube.Because the bias flow is small, the wash flow purging the tube will besmall. In such a case, not all of the retrograde volume will be washedout before the next inspiration. As a consequence, the overall inspiredgas will have a somewhat reduced oxygen concentration and an elevatedcarbon dioxide concentration.

FIGS. 15–17 illustrate the flow in the tube between a blower and apatient interface. FIG. 15 is a graph that illustrates breathing airflow in the tube of a conventional CPAP system. Note that during theexhale portion, the flow from the blower to the patient interface alwaysoverpowers the exhale pressure such that there is no retrograde flowinto the tube. This is typically done by setting the air blower pressureand exhaust port resistance such that bias flow out of the patientinterface is relatively high and the possibility of retrograde flow isavoided. This normal flow occurs even for the overbreathing associatedwith central sleep apnea.

FIGS. 16 and 17 are diagrams that illustrate the effect of breathing insystems of the present invention in which the blower pressure and biasflow out of the exit hole of the patient interface are set such thatthere is retrograde flow during portions of overbreathing associatedwith central sleep apnea.

FIG. 16 illustrates the situation in which there is normal breathing.Even with normal breathing, there is some retrograde flow during theperiod 202. Later in the exhale period the retrograde volume is washedfrom the tube by the normal flow that occurs during period 204. Thusthere is little or no rebreathing during the normal breathing periods.The system of the present invention does not add dead space during thenormal breathing periods. This is important because the addition of deadspace can increase the concentration of carbon dioxide that is suppliedto the bloodstream. It is assumed that if the increased carbon dioxidelevel persists for multiple days, the body will readjust the internalfeedback system an undesirable manner.

FIG. 17 illustrates an embodiment showing overbreathing along with theapparatus of the present invention. In the embodiment of FIG. 17, theoverbreathing is such that there is some retrograde flow of exhaledgases, which remain in the tube at the time of the next inhale portion.This means that at the next inhale portion, the patient will reinspiresome exhaled gases with the resultant higher concentration of carbondioxide. Note that in FIG. 17, the initial exhale region 206 is greaterthan the exhale region 208.

In one embodiment of the present invention, the retrograde flow volumeand wash volume for the normal breathing can be used to set theoperation of the present invention. In one embodiment, the retrogradevolume region 202 should be one-half the size of the wash flow region204 for normal breathing. Other rules of thumb such as the comparisonsof the aveolar ventilation to the bias flow out of the patient interfaceand/or comparisons of the washout time to the duration of expirationcould also be used to set the operations of the system of the presentinvention.

FIG. 4B shows the device of FIG. 4A with the addition of a computer 67and flow meter 69. The flow meter 69 is used to detect the desired airflow in the tube 62. The blower can then be adjusted so that there isretrograde flow during periods of overbreathing and no retrograde flowotherwise. The device of FIG. 4B can be used to calibrate the device ofFIG. 4A for an individual patient.

FIGS. 18A and 18B illustrate an embodiment in which the patientinterface exit size is slowly changed over the course of the night. Inthis embodiment, the blower 210 supplies airflow at a selected pressure.Flowmeter 212 is connected into the tube 214 which allows the flow inthe tube 214 to be determined along with additional parameters of thesystem including the aveolar volume, bias flow, and the like. Theprocessor 216 slowly changes the size of the exit hole using thevariable air resistance apparatus 218. Unlike the system of FIG. 2, thesize of the variable output resistance 218 is modified slowly over thenight.

Looking at FIG. 18B, if the patient has obstructive sleep apnea as wellas central sleep apnea, at the beginning of the night the output valvecan be set relatively large, increasing the bias flow out of the patientinterface and thus reducing any effect of retrograde into the tube 214.Once the obstructive sleep apnea is reduced, the valve diameter can beslowly decreased, which can cause an increase of retrograde flow intothe tube 214 during the overbreathing portion of central sleep apnea andthus can cause rebreathing which can reduce the central sleep apnea.Additional adjustments in the patient interface valve opening can bemade based upon calculations made by the processor 216.

FIG. 5 is an embodiment of the present invention in which the blower 70is dynamically controlled. A flow meter 72 is placed in the tube 74between the blower 70 and patient interface 76. A flow meter can also beplaced near the patient's face. The system of FIG. 5 allows computercontrol to decrease the blower pressure during certain periods of asleep cycle. Thus, during periods of heavy breathing, the blowerpressure can be reduced to facilitate retrograde flow and rebreathing.This embodiment is less advantageoous because of the mixed effects ofchanges in the patient interface pressure. By modifying the gas supplypressure supplied by the blower 70, the retrograde flow into the tube 74can be increased and decreased, as desired.

FIG. 6 is an alternate embodiment of the present invention. In thisembodiment, the patient interface 82 is connected to dead space 84 bycomputer-controlled valves 86 and 88. The amount of rebreathing duringcertain period of the sleep cycle can be modified by changing bias flowby opening and closing the valves 86 and 88, thus reducing the centralsleep apnea.

FIG. 7 is an embodiment using a recirculator 90. During certain portionsof the sleep cycle, the recirculator 90 allowing exhaled air to be drawnin by the recirculator 90 recirculated and supplied to the user at thepatient interface 92. In this manner, the central sleep apnea can bereduced by increasing the rebreathing at selected portions of the sleepcycle.

Technical Description

One embodiment of the invention is applied in the setting of nasalcontinuous positive pressure (CPAP) therapy. The loop gain of thenegative feedback respiratory control system is reduced principally byincreasing the volume of external dead space (V_(ED)), the common airwaythrough which gas is conducted during inspiration and expiration. Theexternal dead space constitutes an extension of the internal dead space(V_(ID)) comprising the airways of the lung and the upper airway. Thetotal dead space (V_(D)) equals the sum of the internal and externaldead spaces.V _(D) =V _(ED) +V _(ID)  (Equation 1)

This volume represents an obligatory inefficiency of the control systemin that it reduces the portion of the tidal volume (V_(T)) thatparticipates in gas exchange within the lungs. Specifically, the tidalvolume is the sum of two componentsV _(T) =V _(D) +V _(A)  (Equation 2)where V_(A) represents the “alveolar” portion of the tidal volume, i.e.the volume that participates in respiratory gas exchange. Also, V _(E)=V_(A)+V _(D), where the symbols V _(E), V _(A), and V _(D) signify theproducts f.V_(T), f.V_(A) and f.V_(D) (f represents respiratoryfrequency). In the negative feedback loop of the respiratory controlsystem (FIG. 1), V _(E) represents the output of the respiratory centralpattern generator and V _(A) is a variable which influences arterialblood gas pressures. The link between V _(E) and V _(A) is, of course, V_(D) which is the primary variable manipulated in dynamicallycontrolling loop gain.

Dynamic control of the rebreathing volume is achieved when the patientis breathing through a nasal CPAP apparatus. When using conventionalnasal CPAP the nose is covered by a mask which is connected to apressure-generating source by a length of tubing. The nose mask isflushed continuously by a stream of gas flowing from the pressure sourceand exiting the exhaust port of the mask. This will be referred to asthe bias flow (V _(B)). When using nasal CPAP for its traditionalapplication, i.e., treatment of OSA, the rate of exhaust flow isrelatively high so that virtually all the expired gas which enters themask from the nose flows into the mask and out the exhaust port. Becauseof the relatively high V_(B) the mask is completely washed out beforethe next inspiration occurs. Thus, the gas inspired from the mask has acomposition equal to that flowing from the blower (typically room air:F_(|O2)=0.293; F_(|CO2)=0.0003). In this situation, typical for OSAtreatment, the nose mask adds no external dead space. The inventiondynamically increases V_(ED) by using a lower value of V _(B) and this,in turn, dynamically reduces V _(A) (Equation 2). Thus, the component ofpulmonary ventilation effective in gas exchange, alveolar ventilation (V_(A)), is altered on a moment-to-moment basis. Since V _(A) determinesthe values of the feedback variables, arterial P_(O2) and P_(CO2), V_(D) directly influences loop gain (FIG. 1). Thus, the loop gain (L.G.)of the system can be manipulated as below:↓V _(B)→↑V_(ED)→↓V _(A)→↓L.G.  (Equation 3)Importantly, the increase in V_(ED) occurs only during periods ofhyperpnea, as described below. Thus, during normal breathing, no deadspace is added to the system.

As secondary strategies, the invention utilizes changes in CPAP pressureto change lung volume and, thereby, influence loop gain of therespiratory control system. In particular, in increase in lung volumedecreases loop gain by decreasing the dynamic change in feedbackvariables (arterial P_(CO2) and P_(O2)) when alveolar ventilationchanges dynamically. As well, such an increase in lung volume decreasesthe end-expiratory length of inspiratory muscles, thereby decreasingtheir force generation during inspiration. Together, both effects ofnasal CPAP decrease the loop gain. When CPAP pressure is dynamicallyvaried in synchrony with the periodic breathing cycle, both effectsdynamically modulate loop gain. However, experience indicates that, overthe range of CPAP pressure of 1–10 cm H₂O, these produce a smallerdecrease in loop gain than varying V_(D). Additionally, dynamic changesin V_(D) are less likely to disturb the sleeper than changes in CPAPpressure. Accordingly, the use of increase in CPAP pressure to decreaselung volume and, thereby, decrease loop gain, represents a supplementarystrategy of the present invention.

The patient with central sleep apnea or combined central and obstructivesleep apnea sleeps with a nasal CPAP mask sealed to the face (FIGS. 2,4A, 4B, 5, 6, 7). Mouth leaks, if present, are eliminated by a chinstrap and/or an oral appliance combined with nasal occlusion. If this isnot adequate, the nose mask is replaced with a full face mask. Thepatient interface is connected to a positive pressure outlet of a lowimpedance blower by a tubing, in one embodiment typically 2–3 cm indiameter and 1.5 m long. The bias flow exits the patient interfaceeither through an orifice of fixed, selectable size (FIGS. 4A, 4B, 5, 6,7) or through a tubing, in one embodiment (1.5 m long, 1 cm in diameter)connected to a computer-controlled variable resistor (FIG. 2). In such asystem, the patient interface pressure is determined by blower RPM, andthe rate of bias flow V _(B) is the resultant of patient interfacepressure and patient interface outflow resistance. The apparatus shownin FIGS. 2 and 4B includes a pneumotachagraph for measuring flow fromthe blower. This device is suitable for initial titration or for nightlytherapeutic use. Also, a CO₂ meter can be added with a sampling catheterconnected to the patient interface. This allows monitoring of end-tidaland inspired F_(CO2). The device shown in FIG. 4A is a simpler versionof that shown in FIG. 4B and is suitable for nightly use.

The dynamically variable bias flow device (FIG. 2) allowsmoment-to-moment adjustment of bias flow with negligible changes inpatient interface pressure. The exhaust resistor can be controlled by anindependent observer during a polysomnographic study, or it can beautomatically controlled by a computer algorithm. The control ofexternal dead space volume (V_(ED)) is either passively adjusted withthe exhaust resistance being constant, or actively adjusted with exhaustresistance being varied in time. In the passive adjustmentimplementation, bias flow is constant in time since a fixed exhaustorifice is used. In the active adjustment, bias flow changes in timeowing to the change in resistance of the bias flow resistor.

FIG. 8 depicts airflow recorded in the tubing which connects the blowerto the patient interface. Positive values signify airflow from theblower to the patient interface, and negative values indicate airflowfrom the patient interface to the blower. The former is referred to as“wash” airflow since it eliminates expired gas from the patientinterface; the latter is referred to as “retrograde” airflow since itrepresents expired air flowing in the reverse direction to that whichnormally occurs during CPAP administration. As shown in FIG. 8A (toppanel), airflow in the tubing is equal to the sum of two air flows, V_(B) and respiratory airflow. The former is constant and the lattervaries with the respiratory cycle. Inspiratory airflow produces anupward deflection in V and expiratory airflow produces a downwarddeflection in V. At the end of expiration (upward arrow in FIG. 8A),respiratory airflow equals zero and tubing airflow equals bias airflowwhich is chosen to be 1.0 L/sec in this example. Peak expiratory airflowoccurs early in expiration (downward arrow in FIG. 8A) and equals 1.0L/sec in this example. At this time, tubing airflow is zero because peakexpiratory airflow equals V _(B).

FIGS. 8A, 8B, 8C, 8D, 8E and 8F depict the changes in tubing airflowthat occur as V _(B) is progressively reduced from 1.0 L/sec (FIG. 8A)to 0.15 L/sec (FIG. 8F). Respiratory airflow is held constantthroughout. As V _(B) is reduced from 1.0 to 0.5, 0.35, 0.25, 0.20 and0.15 L/sec (FIGS. 8A, 8B, 8C, 8D, 8E, 8F), retrograde airflow appearsduring expiration and becomes progressively larger. The volume of airwhich moves retrogradely during expiration (V_(ret), hatched area)increases progressively as V _(B) is decreased. Conversely, the volumeof air which moves from the blower to the patient interface duringexpiration (V_(wash), stippled area) decreases as V _(B) is decreased.

The volume of air resident in the patient interface and tubing at theend of expiration (downward arrow, FIG. 8A) is referred to as residualvolume (V_(R)). V_(R) can be estimated as the difference betweenV_(RET)−V_(WASH).V _(R) =V _(RET) −V _(WASH)  (Equation 4)In the first five examples shown in FIG. 3, V_(R) is negative or equalto zero (FIGS. 8A, 8B, 8C, 8D, 8E), signifying that with thisrespiratory pattern, there is no added dead space (V_(ED)=0). However,if pulmonary ventilation were to increase, V_(RET) would increase andV_(R) would become positive. Similarly, if the duration of expiration(T_(e)) were to decrease, V_(WASH) would decrease and V_(R) would becomepositive. When breathing is stimulated by an increase in arterialP_(CO2) and a decrease in arterial P_(O2), tidal volume increases andT_(e) decreases. Accordingly, if V_(B) is relatively low (0.35 and 0.25in this example), chemical stimulation will cause V_(R) to assume apositive value so that higher levels of pulmonary ventilation will beassociated with greater values of V_(R).

The presence of a positive value for V_(R) indicates that V_(ED) willassume a finite value (FIG. 8F). However, V_(R) does not equal V_(ED).During inspiration, gas resident in the patient interface and tubingflows to one of two places, namely: out the exhaust port or into therespiratory tract. Only the latter constitutes V_(ED). Accordingly, afraction of V_(R) will be inspired, that fraction depending on the valueof V _(B) relative to the inspiratory flow rate. Use of a high value ofV _(B) will minimize V_(ED). Thus, chemical stimulation of breathingcauses three changes in the respiratory pattern, an increase inexpiratory air flow rate, a decrease in Te, and an increase ininspiratory air flow rate, each of which acts independently to augmentV_(ED). Together, they cause a sharp rise in V_(ED) when V_(E) increasesby chemical stimulation if the V _(B) is relatively low. FIG. 8Billustrates the time in expiration when the tubing and patient interfaceare flushed by fresh, room air. This time is expressed as a fraction ofT_(e) and referred to as T_(FRAC). T_(FRAC) increases progressively as V_(B) decreases. When T_(FRAC) equals 100%, a critical value of V _(B)has been reached; further decreases in V _(B) will produce a finitevalue of V_(ED).

To calculate V_(ED), the following relationship is used:V _(ED) =V _(R)−( V _(B))(t′)  (Equation 5)where t′ defines the time required for V_(R) to be eliminated from thepatient interface and conducting tubing as shown in FIG. 9. V_(ED) canbe calculated by progressively incrementing inspiratory time (t) fromzero (the onset of inspiration) and calculating V_(SUM) inspired volumeplus exhaust port volume, i.e.,V _(SUM)=∫₀ ^(t) V ₁+∫₀ ^(t) V _(B)  (Equation 6)where V₁ represents inspiratory flow rate, i.e., total flow minus biasflow during inspiration. The incrementing procedure continues untilV_(SUM) equals V_(R).

FIG. 9 depicts the changes in V_(RET) and V_(ED) that occur whenpulmonary ventilation is stimulated by increasing arterial P_(CO2). V_(B) is assumed to equal 0.25 L/sec in all cases, and is approximatelytwo times resting V _(A)(5.7 L/min). FIG. 8D depicts the respiratorypattern under unstimulated, resting conditions (V _(E)=8.0 L/sec). Whenventilation is mildly stimulated (V _(E)=15.0 L/sec, FIG. 9A), V_(RET)increases and V_(WASH) decreases so that V_(ED) equals 0.26 L. Furtherstimulation of breathing (FIG. 9B) results in V_(ED) equal to 0.47 Lwhen V _(E) equals 19.5 L/sec, V_(ED) equal to 0.79 L when V _(E) equals25.7 L/sec (FIG. 9C) and V_(ED) equal to 1.19 L when V_(E) equals 36.7L/sec (FIG. 9D). Note that T_(FRAC) increases progressively as V _(E)increases for a constant V _(B).

The dependence of V_(RET), V_(ED) and T_(FRAC) on V _(E) is shown inFIGS. 10, 11 and 12, respectively, for all four values of V _(E). Eachplot shows a family of V _(B) isopleths. V_(RET), V_(D) and T_(FRAC)show a quasi-linear increase as V_(E) increases (FIGS. 10, 11, 12 and13).

FIG. 13 illustrates the relationship between V _(A) and V _(E) at thefive levels of V _(B). For values of 1.0 L/sec and greater, all pointslie on a monotonically ascending curve. However, for lower values of V_(B), the relationship is shifted downward, indicating that an incrementin V _(E) caused by an increase in chemical stimulus will cause asmaller increment in V _(A). This implies a reduction in loop gain whichcan be quantitated as the change in slope of this relationship. Notethat at values of V _(E) equal to 0.35 L/sec less, V _(A) becomesconstant for values of V _(E) greater than 15 L/sec. In other words, theinvention clamps V _(A) at some maximal value.

FIG. 14 illustrates the overall dependence of loop gain on the ratio,log V _(E)/V _(A). This ratio, calculated for resting breathing,provides a normalized index of V _(E) for any patient. The relationshipis plotted over the range of log V _(E)/V _(A) from 0 to 1., i.e. overthe range of variation in V _(E) where V_(ED) is less than zero underresting conditions. Note that the loop gain decreases steeply as restingV_(RET)/V_(WASH) decreases from 0.5 to 0. For this reason, we select aratio value of 0.3 for usual application of the method in treatingcentral sleep apnea. In this situation, V _(B) is approximately twotimes V _(A) and T_(FRAC) equals 80%. This value results in a 50%decrease in loop gain while providing more than adequate washout ofexpired gases from the apparatus under resting conditions. Accordingly,loop gain is reduced to a value that stabilizes breathing for manypatients with central sleep apnea without any risk of adding externaldead space when the patient is breathing normally and having no centralsleep apnea.

The goal of the passive dead space method is to apply nasal CPAP with aV _(B) sufficient to produce V_(ED)=0 under resting conditions, but suchthat the V_(ED) will increase with increasing V _(E) sufficient toreduce the loop gain and stabilize breathing. Specifically, duringhypopnea or normal breathing, the apparatus produces no gas exchangeinefficiency in breathing. However, during hyperpnea, V _(ED) increasesprogressively as V _(E) rises above normal. The net effect is that V_(D)is dynamically adjusted in keeping with variations in V _(E) such thatthe periodic fluctuation in V _(A) is attenuated. This means thatfluctuations in arterial P_(O2) and P_(CO2) are reduced, so that loopgain of the system is reduced. This acts to stabilize breathing.

The advantage of the passively adjusting dead space device is that loopgain can be reduced by a relatively simple apparatus requiring no activealgorithmic, dynamic adjustment in V _(B). Once the effective V _(B) hasbeen determined, this can be achieved by permanent adjustment of theresistance of the exhaust port of the patient interface, therebyeliminating the need for an exhaust tubing and computer-controlledexhaust resistor. However, if the loop gain of the patient's respiratorycontrol system is very high, the passive apparatus may not reduce theloop gain sufficiently to stabilize breathing. In that case, adynamically adjusting V_(ED) apparatus is employed. In the embodimentthat dynamically adjusts V _(B), the indicator variables (V_(RET),V_(WASH), T_(frac) and V_(B)/V_(A)) are calculated on line. Periodicbreathing is detected either by the recurrence of apneas or byautoregressive analysis. V _(B) is reduced progressively until evidenceof central sleep apnea is eliminated or until the indicator variablesreach their critical limits (V_(RET), V_(WASH)=0.8, T_(frac)=80% and V_(B)/V _(A)=2).

FIG. 19 depicts another embodiment of the invention. The patient withcentral sleep apnea wears a full face mask 220 which can be loosefitting, but should be leak resistant such as by using an oral interfacewith nasal occlusion. The mask 220 is purged by a bias flow from ahigh-impedance blower 222 which supplies a constant rate of airflow tothe mask. This bias flow is selectable and rapidly adjustable by thecontrolling computer 224. The bias flow exits to the atmosphere througha low-resistance reservoir tubing 226. The respiratory airflow (bothinspiration and expiration) occurs through this reservoir tubing.Because of the tubing's low resistance, the mask pressure remains nearatmospheric pressure. A pneumotachograph (flow meter 228) in thereservoir tubing allows monitoring of bias flow and respiration airflowand calculation of wash volume during expiration and expired tidalvolume.

Under resting conditions or when no central sleep apnea respiration isdetected, bias flow is held relatively high so that wash volume exceedsthe volume of gas expired into the tube. Accordingly, when inspirationbegins, the reservoir tube has been washed completely with bias flow,and the patient inspires room air. Thus, no external dead space has beenadded when the patient is breathing normally and no ventilatoryperiodicity is detected by the computer. When the computer 222 detectsventilatory periodicity, bias flow is varied in synchrony with theperiodicity. Specifically, when instantaneous ventilation is greaterthan the moving average, bias flow is reduced so that wash volume isless than expired tidal volume. This causes rebreathing and decreasesloop gain of the system. During periods of underbreathing, bias flow ismaintained at high values so that no rebreathing occurs. The volume ofgas resident in the reservoir tubing 226 at the end of expiration (i.e.,the rebreathing volume) is calculated on line and is adjusted to beproportional to the difference between instantaneous ventilation andmoving average ventilation. Thus, dead space increases progressively asoverbreathing occurs, thereby minimizing the effect of the excessiveventilation on arterial blood gases. This, in turn, minimizes theduration of the apnea or magnitude of hypopnea that follows theoverbreathing and stabilizes ventilation.

It will be appreciated by those of ordinary skill in the art that theinvention can be implemented in other specific forms without departingfrom the spirit or central character thereof. The presently disclosedembodiments are therefore considered in all respects to be illustrativeand not restrictive. The scope of the invention is indicated by theappended claims rather than the foregoing description, and all changeswhich come within the meaning and range of equivalence thereof areintended to be embraced herein. Accordingly, the above description isnot intended to limit the invention, which is to be limited only by thefollowing claims.

1. An apparatus for treating a breathing disorder comprising: a gassupplying means; and a leak resistant patient interface operablyconnected using a tube to the gas supplying means, the leak resistantpatient interface having an exit, wherein the apparatus is arranged suchthat during periods of increased breathing associated with the breathingdisorder, some exhaled gasses from the patient flow retrograde into thetube towards the gas supplying means and away from the exit; wherein theapparatus is adapted such that during an initial exhale portion ofincreased breathing associated with the breathing disorder, some exhaledgasses from the patient flow retrograde into the tube towards the gassupplying means and away from the exit and wash flow out of the tubesuch that during a next inhale portion a sufficient amount ofrebreathing occurs to control the breathing disorder.
 2. The apparatusof claim 1, wherein the leak resistant patient interface comprises anoral interface and nasal occlusion device.
 3. The apparatus of claim 2,wherein the apparatus is adapted such that during normal breathingperiods little rebreathing occurs.
 4. The apparatus of claim 3, whereinthe apparatus is adapted such that during normal breathing periods someretrograde flow occurs but wash flow is sufficient to remove exhaled airbefore a next inhale portion.
 5. The apparatus of claim 4, wherein theretrograde flow into the tube is influenced by gas pressure from the gassupplying means and by an exit hole size.
 6. The apparatus of claim 1,wherein gas pressure from the gas supplying means is set below four cmH₂O pressure.
 7. An apparatus for treating a breathing disordercomprising: a gas supplying means; and a leak resistant patientinterface operably connected using a tube to the gas supplying means,the leak resistant patient interface having an exit, wherein theapparatus is arranged such that during periods of increased breathingassociated with the breathing disorder, some exhaled gasses from thepatient flow retrograde into the tube towards the gas supplying meansand away from the exit, wherein gas pressure from the gas supplyingmeans is set at a controlled level below four cm H₂O pressureindependently of the respiratory cycle of the patient.
 8. The apparatusof claim 7, wherein the leak resistant patient interface comprises anoral interface and nasal occlusion device.
 9. The apparatus of claim 8,wherein a size of the exit size is adjustable.
 10. The apparatus ofclaim 9, wherein pressure in the leak resistant patient interface is sethigh enough to treat obstructive sleep apnea.
 11. The apparatus of claim10, wherein the gas-supplying means comprises a blower which blows airto the leak resistant patient interface.
 12. The apparatus of claim 7,wherein the leak resistant patient interface is adapted to fit about apatient's nose.
 13. The apparatus of claim 7, wherein the gas-supplyingmeans is adjustable.
 14. A method of treating a patient suffering from abreathing disorder, the method comprising: providing an apparatuscomprising a gas supplying means and a leak resistant patient interfaceadapted to be fit on the patient's airway, the leak resistant patientinterface operably connected using a tube to the gas supplying means,the leak resistant patient interface having an exit; fitting the leakresistant patient interface to the patient; and adjusting the apparatussuch that during periods of increased breathing associated with thebreathing disorder, some exhaled gasses from the patient flow retrogradeinto the tube, wherein the adjusting step is done such that during aninitial exhale portion of increased breathing associated with thebreathing disorder, some exhaled gasses from the patient flow retrogradeinto the tube and wash flow out of the tube such that during a nextinhale portion some rebreathing occurs sufficient to treat the breathingdisorder.
 15. The method of claim 14, wherein the leak resistant patientinterface comprises a dental appliance and a nasal occlusion device, andfitting the leak resistant patient interface to the patient comprises:fitting the dental appliance to the mouth of the patient; and blockingthe patient's nose with the nasal occlusion device.
 16. The method ofclaim 15, wherein the adjusting step is done such that during normalbreathing periods little rebreathing occurs.
 17. The method of claim 16,wherein the adjusting step is done such that during normal breathingperiods some retrograde flow occurs but wash flow is sufficient toremove exhaled air before a next inhale portion.
 18. The method of claim15, wherein the retrograde flow into the tube is influenced by gaspressure from the gas supplying means and by an exit hole size.
 19. Themethod of claim 14, wherein the adjusting step is such that gas pressurefrom the gas supplying means is set below four cm H₂O pressure.
 20. Amethod of treating a patient suffering from a breathing disorder, themethod comprising: providing an apparatus comprising a gas supplyingmeans and a leak resistant patient interface adapted to be fit on thepatient's airway, the leak resistant patient interface operablyconnected using a tube to the gas supplying means, the leak resistantpatient interface having an exit; fitting the leak resistant patientinterface to the patient; and adjusting the apparatus such that duringperiods of increased breathing associated with the breathing disorder,some exhaled gasses from the patient flow retrograde into the tubewherein the adjusting step is such that gas pressure from the gassupplying means is set at a level below four cm H₂O pressureindependently of the respiratory cycle of the patient.
 21. The method ofclaim 20, wherein the leak resistant patient interface comprises adental appliance and a nasal occlusion device, and fitting the leakresistant patient interface to the patient comprises: fitting the dentalappliance to the mouth of the patient; and blocking the patient's nosewith the nasal occlusion device.
 22. The method of claim 21, wherein theadjusting step includes adjusting an exit hole size.
 23. The method ofclaim 21, wherein pressure in the leak resistant patient interface isset high enough to treat obstructive sleep apnea.
 24. The method ofclaim 21, wherein the gas supplying means comprises a blower.
 25. Anapparatus for treating a breathing disorder comprising: a gas supplyingmeans; a leak resistant patient interface adapted to be fit on apatient's airway, the leak resistant patient interface operablyconnected using an input tube to the gas supplying means, the leakresistant patient interface having an exit; a variable air resistancemeans operably connected to the exit of the leak resistant patientinterface; and a controller operably connected to the variable airresistance means to adjust a level of rebreathing that occurs andmaintain a temporally variable flow of air in the input tube withoutproducing significant deviations in leak resistant patient interfacepressure.
 26. The apparatus of claim 25, wherein the leak resistantpatient interface comprises an oral interface and nasal occlusiondevice.
 27. The apparatus of claim 26, wherein the variable airresistance means comprises an adjustable valve.
 28. The apparatus ofclaim 27, further comprising an exit tube between the leak resistantpatient interface and the adjustable valve.
 29. The apparatus of claim26, wherein the controller adjusts the variable air resistance means toprovide a dead space during certain portions of a sleep cycle.
 30. Theapparatus of claim 26, wherein the controller adjusts the variable airresistance means to modify exit flow out of the leak resistant patientinterface at different times during a night's sleep.
 31. The apparatusof claim 26, further comprising a flow meter adapted to provide signalsto the controller.
 32. The apparatus of claim 26, wherein the controllerdetects periodicities in sleep cycle to determine how to adjust thelevel of rebreathing.
 33. An apparatus for treating a breathing disordercomprising: a blower; a leak resistant patient interface adapted to befit on a patient's airway, the leak resistant patient interfaceincorporating a dental appliance to reduce mouth leaks and a nasalocclusion device to eliminate nose leaks, the leak resistant patientinterface operably connected using a tube to the blower; and a processoradapted to adjust a level of rebreathing to control the breathingdisorder in the patient by adjusting an active control element of theapparatus.
 34. The apparatus of claim 33, wherein the active controlelement is a variable air resistance means operably connected to a exitof the leak resistant patient interface.
 35. The apparatus of claim 33,wherein the active control element is a unit to adjust a blower output.36. The apparatus of claim 35, wherein the active control element is aunit to adjust a blower output revolutions per minute.
 37. The apparatusof claim 35, further comprising an exit tube connected to the leakresistant patient interface.
 38. The apparatus of claim 33, wherein theactive control element is a recirculator.
 39. The apparatus of claim 33,wherein the active control element is a valve to a dead space volume.40. The apparatus of claim 33, wherein the active control element isadjusted during a periodic sleep cycle of the patient.
 41. The apparatusof claim 33, wherein the active control element is adjusted over anentire sleeping period.
 42. The apparatus of claim 33, wherein theprocessor receives data from a flow meter or a carbon dioxide sensor.43. An apparatus for treating a breathing disorder comprising: a blower;and a leak resistant patient interface adapted to be fit on a patient'sairway, the leak resistant patient interface operably connected using atube to the blower, the leak resistant patient interface having an exit,the resistance of the exit being set such that during treatment of thebreathing disorder in the patient, expiratory air from the patient flowsthrough the tube towards the blower and away from the exit, wherein theapparatus is arranged such that a gas flow from the blower is less thanthat used to treat obstructive sleep apnea.
 44. The apparatus of claim43, wherein the leak resistant patient interface comprises an oralinterface and nasal occlusion device.
 45. The apparatus of claim 44,wherein gas pressure from blower is set at four cm H₂O pressure orbelow.
 46. The apparatus of claim 44, wherein the apparatus is arrangedsuch that during periods of increased breathing associated with thebreathing disorder, some exhaled gasses from the patient flow retrogradeinto the tube.
 47. The apparatus of claim 46, wherein the apparatus isadapted such that during an initial exhale portion of increasedbreathing associated with the breathing disorder, some exhaled gassesfrom the patient flow retrograde into the tube and wash flow out of thetube such that during a next inhale portion some rebreathing occurs. 48.The apparatus of claim 46, wherein the apparatus is adapted such thatduring normal breathing periods little rebreathing occurs.
 49. Theapparatus of claim 46, wherein the apparatus is adapted such that duringnormal breathing periods some retrograde flow occurs but wash flow issufficient to remove exhaled air before a next inhale portion.
 50. Theapparatus of claim 44 wherein the blower is adjustable.
 51. A method oftreating a patient suffering from a breathing disorder, the methodcomprising: providing an apparatus comprising a blower and a leakresistant patient interface adapted to be fit on the patient's airway,the leak resistant patient interface operably connected using a tube tothe blower, the leak resistant patient interface having an exit; fittingthe leak resistant patient interface to the patient's airway; andadjusting the apparatus such that gas flow from the blower is controlledat a variable flow rate and essentially constant pressure, the pressurebeing less than that used to treat obstructive sleep apnea, in order totreat the breathing disorder in the patient.
 52. The method of claim 51,wherein the leak resistant patient interface comprises a dentalappliance and a nasal occlusion device, and fitting the leak resistantpatient interface to the patient comprises: fitting the dental applianceto the mouth of the patient; and blocking the patient's nose with thenasal occlusion device.
 53. The method of claim 52, wherein theadjusting step is such that gas pressure from the blower is set belowfour cm H₂O pressure.
 54. The method of claim 52, wherein the adjustingstep is such that during periods of increased breathing associated withthe breathing disorder, some exhaled gasses from the patient flowretrograde into the tube.
 55. The method of claim 54, wherein theadjusting step is done such that during an initial exhale portion ofincreased breathing associated with the breathing disorder, some of thepatient's exhaled gasses flow retrograde into the tube and wash flow outof the tube such that during a next inhale portion some rebreathingoccurs.
 56. The method of claim 54, wherein the adjusting step is donesuch that during normal breathing periods little rebreathing occurs. 57.The method of claim 56, wherein the adjusting step is done such thatduring normal breathing periods some retrograde flow occurs but washflow is sufficient to remove the exhaled air before a next inhaleportion.
 58. The method of claim 54, wherein the retrograde flow intothe tube is influenced by gas pressure from the blower and by a size ofthe exit.
 59. A method comprising: providing an apparatus comprising ablower and a leak resistant patient interface adapted to be fit on apatient's airway, the leak resistant patient interface operablyconnected using a tube to the blower, the leak resistant patientinterface having an exit, the resistance of the exit being set thatduring treatment of a breathing disorder in the patient, expiratory airfrom the patient flows through the tube towards the blower and away fromthe exit; fitting the leak resistant patient interface to the patient'sairway; treating an obstructive sleep apnea with the apparatus;adjusting the apparatus to treat the breathing disorder; and treatingobstructive sleep apnea with the apparatus.
 60. The method of claim 59in which the leak resistant patient interface comprises a dentalappliance and a nasal occlusion device, and fitting the leak resistantpatient interface to the patient comprises: fitting the dental applianceto the mouth of the patient; and blocking the patient's nose with thenasal occlusion device.
 61. The method of claim 60, wherein theadjusting step comprises adjusting the apparatus such that gas flow fromthe blower is less than that used to treat obstructive sleep apnea totreat the breathing disorder in the patient.
 62. The method of claim 60,wherein the adjusting step is such that gas pressure from the blower isset below four cm H₂O pressure.
 63. The method of claim 60 wherein theadjusting step is such that during periods of increased breathingassociated with the breathing disorder, some of the patient's exhaledgasses flow retrograde into the tube.
 64. The method of claim 63,wherein the adjusting step is done such that during an initial exhaleportion of increased breathing associated with the breathing disorder,some exhaled gasses from the patient flow retrograde into the tube andwash flow out of the tube such that during a next inhale portion somerebreathing occurs.
 65. The method of claim 63, wherein the adjustingstep is done such that during normal breathing periods littlerebreathing occurs.
 66. The method of claim 65, wherein the adjustingstep is done such that during normal breathing periods some retrogradeflow occurs but wash flow is sufficient to remove exhaled air before anext inhale portion.
 67. The method of claim 60, wherein the retrogradeflow into the tube is influenced by gas pressure from the blower and bya size of the exit hole.
 68. The method of claim 60, wherein theobstructive sleep apnea treating step comprises supplying blowerpressure greater than eight cm H₂O.
 69. The method of claim 60, whereinthe obstructive sleep apnea treating step occurs before the adjustingstep.